Project Summary Neutropenia and related infection are the most important dose limiting toxicities in hematopoietic cell transplantation (HCT) and anti-cancer chemo- and radiotherapy, impacting on quality of life and clinical outcomes, with the potential to cause death. Neutropenia-related pneumonias are involved in 40% infections at a site other than blood alone and usually treated with broad-spectrum antibiotic therapy and G-CSF therapy. However, not all patients respond to these treatments. The long-term goal of this project is to explore an alternative strategy for treating/preventing neutropenia-related pneumonia, namely by enhancing neutrophil function in neutropenic patients. We tried to achieve this by elevating intracellular PtdIns(3,4,5)P3 signaling pathway which has been implicated in various neutrophil functions. We confirmed this concept by showing that augmenting PtdIns(3,4,5)P3 signal by disrupting PTEN enhanced neutrophil function and bacterial clearance, and reduced mortality rate in a murine model of neutropenia-related pneumonia. However, PTEN disruption is associated with tumorigenesis, which rendered it unsuitable as a therapeutic target. Recently we reported that PtdIns(3,4,5)P3 signal in neutrophils can also be elevated by disrupting IP6K1, an enzyme responsible for the synthesis of IP7, a cytosolic molecule that negatively regulates PtdIns(3,4,5)P3 signaling. Importantly, homozygous IP6K1 KO mice were viable and did not display any gross physical or behavioral abnormalities. No tumors of any kind were discovered in these mice. Based on these intriguing results, we hypothesize that disruption of IP6K1 should be a legitimate therapeutic strategy for the treatment of neutropenia-related pneumonia. In the last funding period, we investigated the role of IP6K1 is regulating neutrophil function in bacterial pneumonia. Consistent with the elevated PtdIns(3,4,5)P3 signaling in IP6K1 deficient neutrophils, disrupting the Ip6k1 gene (whole-body KO) or pharmacologically inhibiting IP6K1 activity using a specific inhibitor TNP efficiently and effectively enhanced host bacterial killing capability, minimizing the lung damage caused by both Gram-positive and Gram-negative bacterial pneumonia. Unexpectedly, inhibition of IP6K1 reduced pulmonary neutrophil accumulation. We revealed that IP6K1-mediated inorganic polyphosphate (polyP) production by platelets was essential for infection-induced neutrophil-platelet aggregate (NPA) formation which facilitates neutrophil accumulation in alveolar spaces during bacterial pneumonia. As part of our overall goal to further understand the role of IP6K1 and to design the best strategy to target IP6K1 in neutropenia-related pneumonia, we will continue to examine whether neutrophil-specific IP6K1 disruption is sufficient to induce the elevated host defense (Aim 1). In addition, we will elucidate the role of platelet IP6K1 in regulating pulmonary neutrophil accumulation in neutropenia-related pneumonia (Aim 2). Finally, we will directly investigate whether treatment with IP6K1 inhibitor TNP can alleviate neutropenia-related pneumonia and whether polyP or platelet transfusion can enhance the anti-pneumonia effect of TNP (Aim 3).